This invention relates to fuel cells and more particularly, to fuel cells using hydrogen as a fuel with a reaction catalyst which adsorbs carbon monoxide (CO). This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
Fuel cells generally produce useful electrical energy by providing fuel to an anode and an oxidizer to a cathode. wherein reactions on the electrodes generate a current flow therebetween. The electrodes typically contain a catalyst, e.g. platinum (Pt). to promote the reactions occurring at the electrodes. The efficacy of the catalyst is greatly affected by contaminants which adhere to the surface of the catalyst to block hydrogen adsorption onto the catalyst. More particularly, CO strongly adsorbs onto Pt at temperatures below about 150.degree. C. to effectively poison the hydrogen reaction at low operating temperatures.
It will be appreciated that this CO poisoning occurs at even low levels of CO. FIG. 1 illustrates the performance of a proton exchange membrane (PEM) fuel cell with CO levels from 5-100 ppm (0.0005-0.01%) in the hydrogen fuel stream. A fuel cell which would be useful for commercial applications would preferably be tolerant of CO levels produced in a relatively uncomplicated fuel system. i.e., 100 ppm or greater.
One fuel system under consideration is a methanol reformation system. Methanol is a readily available and easily transportable fuel. A system which is effective to generate a fuel stream from methanol is described in U.S. Pat. No. 4,650,727, issued Mar. 17, 1987, to Vanderborgh et al. and incorporated herein by reference. However, conventional reformers provide a hydrogen fuel stream with greater than 0.05% CO, a substantially unusable concentration in current PEM fuel cells.
Attempts have been made to further reduce the CO concentration, and particularly through a selective oxidation process for the CO. Conventional,. the oxidation of CO to CO.sub.2 occurs in the presence of a catalyst and at temperatures above 150.degree. C., i.e., in the range of 175.degree. C. The output from a selective oxidation process still has CO concentrations up to 0.01%.
PEM fuel cells, which have potential application in mass transportation. are very sensitive to CO poisoning, as shown by the FIG. 1 performance curves. Conventional membranes, such as the Dupont Polymer Nafion.RTM. must contain significant amounts of water to conduct protons from the electrode reactions. Accordingly, PEM fuel cells cannot operate at temperatures over about 100.degree. C., and preferably operate at temperatures around 80.degree. C. At these operating temperatures. CO strongly adsorbs to -he Pt catalyst to poison the fuel cell performance, as shown in FIG. 1.
Phosphoric acid fuel cells can operate at temperatures above 150.degree. C. where the CO adsorption and concomitant poisoning is greatly reduced. However, poisoning effects still occur at somewhat higher CO levels. Further, phosphoric acid systems could benefit from operating at lower temperatures to achieve shorter start-up times, and lower corrosion rates.
Accordingly, it is highly desirable to improve fuel cell performance with levels of CO in the fuel stream which have not heretofore been tolerable. The present invention allows PEM fuel cells to obtain a performance level a wide range of CO concentrations which is substantially improved, approaching the performance levels obtainable with a pure hydrogen fuel stream.
Accordingly, it is an object of the present invention to enable a PEM fuel cell to be used with a methanol fuel source.
It is another object of the present invention to enable a PEM fuel cell to maintain substantially uneffected performance cover a wide range of CO concentrations.
One other object of the preset invention is to improve the CO tolerance of phosphoric acid fuel cells.
Yet another object of the present invention is to substantially restore PEM fuel cell performance with CO levels of 100-500 ppm to the performance level without CO.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.